A number of noteworthy developments have come into confluence to result in state-of-the-art optical fiber communication systems. Wavelength division multiplexed (WDM) systems provide for per-channel bit rates of 2.5 Gb/sec with four channels of 100 GHz separation which, together, constitute a spectral band sufficiently narrow to permit simultaneous amplification of the entire WDM channel set by individual erbium-doped fiber amplifiers (EDFA). Several amplifier spans, each of 100 km and greater length, are combined to require terminals only at distances of 500 km or more.
While some analog operation persists, new system design emphasizes digital transmission, so that capacity is ordinarily discussed in terms of bit rates. It has been recognized that a primary limitation on capacity is spreading of individual bits, caused by chromatic dispersion, i.e., differing wavelength-dependent group velocities for different wavelength components of the spectrum making up the pulse. The dispersion limit on bit-rate, or alternatively on distance between terminals (spans), corresponds with a degree of spreading sufficient to result in overlap between successive bit positions. Chromatic dispersion is lessened by use of laser sources of minimal spectral emission bandwidth. From the fiber standpoint, the fact that the natural material dispersion null point of silica--the basic material of which optical fiber is made--occurs within its 1310 nm transparency window, resulted in emphasis on operation at such a system wavelength.
At this stage, 1310 nm operating systems were loss-limited, so that decreased fiber attenuation would permit increased span length. It was known that the 1550 nm window offered decreased loss, but that significant dispersion at wavelengths in that region would now result in systems that were dispersion-limited, in turn preventing any advantage in span length due to reduced loss for contemplated bit rates, and signaling study of means to lessen dispersion. The desire for operation in the 1550 nm window was satisfied by a new fiber design--"dispersion-shifted fiber" (DSF). This fiber depends upon the combination of two opposite influences--on the combination of "waveguide dispersion" with the naturally occurring silica "material dispersion". Appropriate fiber design increased the magnitude of the waveguide dispersion to offset the positive dependence of the material dispersion at the greater wavelength--so as to "shift" the dispersion null point from 1310 nm to 1550 nm. The objective was attained, DSF replaced unshifted fiber (USF) for sophisticated, long-haul systems.
The desire to increase capacity was largely directed to WDM operation, with its multiple-channel operation, now in the lower-loss 1550 nm window. At the same time, a competing effort looked to the possibility of simultaneous operation in both windows, as a means of increasing capacity. Recognizing the significance of chromatic dispersion as the primary limitation on bit-rate, and in recognition of the universally-acclaimed success of DSF which enabled operation at 1550 nm by nulling dispersion at that wavelength, the effort took the form of attempting to null the dispersion simultaneously at both wavelengths. The result was the dispersion flattened fiber (DFF), which, analogous to DSF, depended on crossover between waveguide dispersion and material dispersion--but now required two crossovers, one within each window.
Emergence of the EDFA, with its passband in the 1550 nm window, increased the importance of moving system operation to this region. That amplifier, with its passband of sufficient width for simultaneously amplifying member channels of a WDM set, offered the first practical WDM operation. (By all reports, this amplifier continues to be preferred, although study has made significant inroads on alternative rare-earth doped devices and also on amplifiers based on Raman effect. See Elec. Lett., Vol. 32, No. 23, pp. 2164-2165 (Nov. 7, 1996).)
The next phase was truly remarkable, effectively ending dominance of DSF, and with it, the DFF with its promise of dual-window operation.
TrueWave.RTM. fiber is one commercial name given the "Non-Zero Dispersion" fiber (NZF) which has largely replaced DSF for state-of-the-art use. U.S. Pat. No. 5,327,516, first teaches that DSF precludes expected high-capacity WDM operation by reason of the very same nulled chromatic dispersion recommending it for highest-capacity single-channel operation. It then identifies four-wave mixing (4 WM) as the primary reason for failure of WDM to meet capacity expectations in DSF systems. 4 WM, a non-linear effect, is due to interaction between pulses of adjoining channels, which, in introducing sum and difference signals, reduces power level of the two interacting channels. It is of particular consequence for usual systems, providing for constant channel-to-channel spacing (for "evenly spaced" channels), in which wavelengths of such spurious signals coincide with carrier wavelengths of other channels of the WDM set, resulting in superimposition and reduced signal-to-noise ratio. It then proposes substitution of NZF with its small but critical range of chromatic dispersion. Intended for long-haul operation at a system wavelength of 1550 nm, the specified dispersion at that system wavelength, is shown to be sufficiently low as between components of a pulse spectrum so as to permit contemplated per-channel bit rates, while at the same time introducing sufficient dispersion to result in periodic phase cancellation between channels of the WDM set, and to lessen the magnitude of transferred signals due to 4 WM. (4 WM requires simultaneous presence of bits in interacting member channels--the attained magnitude of the growing spurious signal increasing for increasing time in phase. Dispersion as between channels limits interaction times, and accordingly sets a limit on the cumulative magnitude of the spurious 4 WM signals.)
The art has continued to progress. With emphasis on broadened transmission bandwidth, amplification is, to lesser extent, satisfied by the traditional EDFA. Raman amplification or, alternatively, amplification based on rare earth dopant, but using praseodymium in lieu of erbium (e.g., the praseodymium-doped fluoride fiber amplifier), shows promise of satisfying the desire for WDM operation within the 1310 nm window. Other alternatives have been investigated--as an example, erbium-doped fluoride glass shows evidence of improved gain flattening over the amplification band in the 1550 nm window.
It is widely recognized that advantages of long haul optical communication systems might continue to be of value for transmission over shorter distances. The trend toward "all optical networking" is expected to result in replacement of copper for local access and in metropolitan systems. Needs in such systems would be served by very large WDM sets--perhaps a hundred or more channels--perhaps before such large capacity is seen in long haul systems.